1. Field of the Invention
The present invention relates to an optical packet switching system in which packet-by-packet optical packet switching is enabled by driving an optical switch according to routing information assigned to an optical packet signal.
2. Description of the Related Art
The technology of switching the path depending on the wavelength in an optical transmission system based on wavelength division multiplexing (WDM) and by employing a wavelength selective switch (WSS) is in practical use. One of the next-generation technologies studied is an optical packet switching system in which the path is switched in smaller units, namely, IP packets (10 Gigabit Ethernet (registered trademark) signals, etc.). Each packet is converted in format into an optical packet and routed by using an ultrahigh-speed optical switch (see e.g., patent document No. 1).
When the transmission is based on IP packets, no significant information is transmitted absent any data so that the bandwidth is wasted accordingly. If the optical packet switching system is realized, however, any idle time in which data is absent can be occupied by another packet. Therefore, the optical packet switching system promises the possibility of dramatically increasing the bandwidth usage efficiency of the transmission path and is envisaged as a technology of the future.    [patent document No. 1] JP 2008-235986
An erbium doped fiber amplifier (EDFA) is an essential optical device in a WDM system. When the distance of transmission or the number of switching stages is increased, resultant loss need be compensated by using an EDFA. The same holds true of the optical packet transmission system.
FIGS. 1A and 1B are graphs comparing an optical signal in optical path transmission and an optical packet signal in optical packet transmission. FIG. 1A shows an optical signal in optical path transmission, and FIG. 1B shows an optical packet signal in optical packet transmission. In the case of optical path transmission, the mark ratio of 50% of an optical signal transmitted is constantly guaranteed. Therefore, the average optical power remains constant. In contrast, in the case of an optical packet, the packet length varies for each packet. Therefore, a time period during which an optical signal occurs and a time period during which an optical signal does not occur are created. In this specification, “packet density” is defined as an indicator of a ratio by which an optical signal occurs. A packet density is defined by packet length/packet interval. The packet density of an optical signal in optical path transmission is 100%. If the packet density varies, the average value of optical power varies. For example, when the packet density drops from 50% to 5%, the average optical power drops by 10 dB, neglecting the power occurring in the absence of an optical signal.
FIGS. 2A and 2B are graphs comparing extinction ratios of optical packet signals occurring before and after optical switching. FIG. 2A shows an optical packet signal input to an optical switch, and FIG. 2B shows an optical packet signal output from the optical switch. The extinction ratio of an optical transmitter for transmitting an optical packet signal differs depending on the transmitter configuration. Generally, the extinction ratio is about 8-20 dB. Meanwhile, the extinction of an optical switch is generally 30 dB or more, which is higher than that of an optical transmitter. Accordingly, the extinction ratio of an optical packet signal output from an optical switch is significantly higher than the ratio occurring before the signal is input, as shown in FIGS. 2A and 2B. As a result, variation in the average optical power of an optical packet signal occurring after the signal is output from the optical switch is larger than the variation occurring before the signal is input to the optical switch.
Normally, the carrier relaxation time of an EDFA is on the order of msec. Therefore, an EDFA cannot control the gain of each optical packet, which lasts on the order of nsec or μsec. Thus, an EDFA controls the gain based on the average optical power obtained by averaging over duration on the order of msec. Therefore, large variation in the average optical power caused by variation in packet density or an optical switch characterized by high extinction ratio is a factor that may result in large fluctuation in the gain of an EDFA. As fluctuation in the gain of an EDFA increases, the peak power of an optical power signal output from the EDFA varies significantly even if the peak power of an optical packet signal input to the EDFA remains constant. This might adversely affect the bit error rate characteristics of the optical receiver for receiving the optical packet signal. The term “peak power” as used in this specification means an optical power level occurring when an optical signal represents “1”.